Photosynthesis and Plant Nutrition: Biology 0610 (Cambridge O Level / IGCSE)

In a school greenhouse, two rows of pepper seedlings are grown side by side in identical trays of nutrient solution, except that each row's solution is missing one different mineral ion.

Row P: the seedlings reach a normal height, and their leaves stay a healthy dark green colour, but very few new leaves appear and the stems and leaves remain thin, as though the plant cannot make enough material for new cell growth.

Row Q: the seedlings also reach a normal height and produce plenty of new leaves, but their older leaves turn pale and yellowish between the veins, while the veins themselves stay a darker green.

Which mineral ion is missing from the nutrient solution given to Row P, and which is missing from the nutrient solution given to Row Q?

A gardener is growing a pumpkin plant from seed, and its vine is now spreading across the ground with several developing pumpkin fruits.

(a) Complete the word equation for photosynthesis, as it takes place in the leaves of the pumpkin plant, by filling in the two raw materials and the two products in the correct spaces:

_______________ + _______________ → _______________ + _______________

(this reaction takes place in the presence of light and chlorophyll) [2]

(b) State where in the pumpkin plant's leaf cells chlorophyll is found, and describe its role in photosynthesis. [2]

(c) As the pumpkin plant grows, the glucose made by photosynthesis in its leaves is used in several different ways. Other than being used immediately in respiration to release energy, state two different fates of this glucose, and for each one explain briefly why it is useful to the growing, fruiting pumpkin plant. [4]

A student wants to find out how light intensity affects the rate of photosynthesis in a sprig of hornwort, a small aquatic plant. The student seals the hornwort inside a flask of water containing a fixed, generous concentration of sodium hydrogencarbonate solution (an extra dissolved source of carbon dioxide), together with a dissolved-oxygen probe connected to a datalogger that automatically records the percentage of dissolved oxygen in the water. The flask sits in a water bath held at a constant 25°C throughout, and is placed under an adjustable LED panel, set in turn to three different brightness settings. Each brightness setting is left running for ten minutes before the increase in dissolved oxygen is read from the datalogger:

(a) Identify the independent variable and the dependent variable in this investigation. [2]

(b) State two variables, other than temperature, that the student has kept constant to make this a fair test. [2]

(c) Describe the relationship shown by the results in the table between the LED brightness setting and the increase in dissolved oxygen. [2]

(d) Explain, in terms of the word equation for photosynthesis, why an increase in dissolved oxygen in the flask indicates that the rate of photosynthesis has increased. [2]

A hobbyist grows basil in a sealed indoor grow tent fitted with adjustable LED lights and a small canister that can enrich the air inside with extra carbon dioxide. With temperature held constant at the basil's optimum throughout, the hobbyist records the rate of photosynthesis of the basil (in arbitrary units) at several LED light intensities, both with the normal (atmospheric) carbon dioxide level and with the canister topping up the air to a raised carbon dioxide level:

(a) Complete the balanced chemical equation for photosynthesis below by writing in the missing formula and the two missing balancing numbers:

___CO₂ + ___H₂O → C₆H₁₂O₆ + ___O₂ [2]

(b) Describe how the rate of photosynthesis at the normal carbon dioxide level changes as the LED light intensity increases from 5 to 45 units. [2]

(c) Explain why raising the carbon dioxide level makes almost no difference to the rate of photosynthesis at a light intensity of 5 units, but makes a large difference at a light intensity of 45 units. [3]

(d) Suggest one factor, other than light intensity, carbon dioxide concentration or temperature, that could still limit the rate of photosynthesis in this basil plant even under otherwise ideal grow-tent conditions. [1]

A cross-section through a privet leaf shows four layers, in order from the upper surface of the leaf down to the lower surface:

1. a thin, transparent, waxy layer that lets light through and reduces water loss from the leaf's surface;
2. a layer of tall, closely packed cells, positioned just beneath layer 1, containing many chloroplasts;
3. a layer of loosely packed, irregularly shaped cells with large air spaces between them;
4. a layer containing pairs of guard cells surrounding small pores.

Which named tissue is layer 2, and why is its position, just beneath the upper surface, an advantage for photosynthesis?

A student wants to show that a pelargonium (geranium) plant needs light to make starch in its leaves.

Two days before the investigation, the student places the whole plant in a dark cupboard, where it stays for 48 hours with no light at all.

The student then attaches a small square of aluminium foil to one leaf, covering both the upper and lower surfaces of a patch in the middle of the leaf, while leaving the rest of the same leaf uncovered. The plant is then placed on a sunny windowsill for six hours.

After six hours, the student removes the leaf, takes off the foil, and tests the whole leaf for starch using iodine solution.

(a) Explain why the student places the plant in the dark for 48 hours before starting the investigation. [2]

(b) Describe how the student should prepare the leaf for the iodine test, from removing it from the plant up to the point of adding iodine solution. Your answer should refer to the use of ethanol. [4]

(c) State the colour the iodine solution would turn on (i) the patch of leaf that was covered by foil and (ii) the rest of the leaf that was not covered, and explain what these two results together show. [2]

A student is investigating whether chlorophyll is needed for a leaf to make starch. They use a variegated leaf from a coleus plant, which has patches of green tissue and patches of cream-white tissue scattered across the same leaf. The plant has already been left in bright light for several hours before the leaf is removed and tested for starch with iodine solution.

(a) The student did not destarch the plant in the dark beforehand, and did not cover any part of this leaf with foil. State one variable that is the same across the whole leaf, and one variable that is different across the leaf, that allow this single variegated leaf to act as its own comparison for this investigation. [2]

(b) Predict which parts of the leaf will turn blue-black with iodine solution, and which will stay orange-brown. [2]

(c) Explain, in terms of chlorophyll, why the cream-white patches do not turn blue-black even though they were exposed to exactly the same light as the green patches. [2]

A researcher measures the rate of photosynthesis of a pond plant at six different water temperatures. Throughout the investigation, light intensity and carbon dioxide concentration are both kept high and constant, so that neither of them limits the rate at any point:

Which statement correctly explains why the rate of photosynthesis falls between 35°C and 45°C?

The diagram below describes, in words, a cross-section through part of a leaf, from the upper surface down to the lower surface:

• a thin, transparent, waxy layer covers the very top of the leaf;
• beneath this is a layer of tall, closely packed cells full of chloroplasts;
• below that is a layer of loosely packed, irregularly shaped cells, with large, interconnected air spaces between them;
• at the very bottom of the leaf, the lower surface contains many small pores, each surrounded by a pair of guard cells.

(a) Name the small pores found in the lower surface of the leaf, surrounded by pairs of guard cells, and state one gas that moves into the leaf and one gas that moves out of the leaf through these pores during daylight. [2]

(b) Name the loosely packed layer of cells with large air spaces between them, and explain how the large air spaces in this layer help photosynthesis to happen efficiently. [2]

(c) The leaf as a whole is broad and thin. Explain one way in which being broad, and one way in which being thin, each help the leaf carry out photosynthesis efficiently. [2]

Which of the following is the correctly balanced symbol equation for photosynthesis?